Cyclometallated iridium complexes have been the focus of research and development in OLED (organic lighting-emitting diode, Ching W. Tang et al, Applied Physics Letters, 1987, 51, 913) display devices over last several years. Those complexes can offer higher efficiency when used as phosphorescent dopants in OLED devices since both singlet and triplet excitons generated by electroexcitation can be harvested by a phosphorescent dopant, while only singlets (25% of total excitons) can be utilized when a fluorescent material is used as a dopant. Tris-cyclometallated iridium complexes have demonstrated such advantage. There exist two stereoisomers in homoleptic tri-cyclometallated iridium complexes such as tris(2-(phenyl)pyridinato, N,C2)iridium (III) (Ir(ppy)3), namely facial and meridional isomers as shown below. The facial isomer has been shown to be more desirable as it has demonstrated higher quantum yield and thermal stability than the corresponding meridional isomer (A. B. Tamayo, et al, J. Am. Chem. Soc. 2003, 125, 7377).

There are continuous efforts to develop new phosphorescent dopants for improving the efficiency and operational stability of OLED devices. Mixed tris-cyclometallated iridium complexes have recently attracted attention of research community and their applications to OLED devices have been demonstrated (T. Igarashi et al, US 2001/0019782 A1; J. Kamatani, et al, US 2003/0068526 A1; S. Akiyama et al, JP2003-192691A). However, the synthesis of those mixed ligand complexes is challenging. The method employed in the prior art involves the reaction of a bis-cyclometallated iridium complex with a third ligand in glycerol at high temperature (usually above 180° C.), which was found to produce an undesirable mixture of different homoleptic and heteroleptic tris-cyclometallated iridium complexes formed from ligand-scrambling side reactions which leads to difficulties in separation and purification of the desired compounds. Recently, a method was developed in U.S. Ser. No. 09/729,207 to prepare mixed tris-cyclometallated iridium complexes in high yields and purity, but the products obtained from that reaction are meridional isomers. Also, it was discovered that some meridional isomers could be isomerized to their facial isomers by applying heat, but the high temperature required (above 180° C.) was accompanied by severe decomposition in some cases.
It is a problem to be solved to provide an improved process for efficiently isomerizing meridional mixed tris-cyclometallated rhodium or iridium complexes to their facial isomers by reducing the extent of side reactions and high temperature degradation.